The present invention relates to a method for the production of silicon nitride ceramics of high density by the reaction sintering method and to the production of silicon nitride-type fine ceramics.
Silicon nitride, similarly to silicon carbide, excels in mechanical strength, resistance to thermal shock, resistance to wear, and chemical stability at elevated temperatures and, therefore, finds extensive utility as a high-temperature structural material.
Production processes for silicon nitride sintered products can be classified into the reaction sintering method and the atmospheric sintering method.
The reaction sintering method effects production by subjecting a shaped body of Si powder to a nitriding reaction in a nitriding atmosphere.
This method has the following merits:
(1) The shaped body can be molded by any of various methods;
(2) Since dimensional contraction after the nitriding reaction is extremely small, products of a large and complicated shape can be obtained relatively easily with high dimensional accuracy;
(3) Since the nitriding reaction generally does not require the use of any sintering additives, the shaped body consequently produced retains strength and other properties intact even at elevated temperatures;
(4) Sintered products of larger size can be produced more easily as compared with the atmospheric sintering method; and
(5) Sintered products obtained by this method show no reduction in strength even under a high temperature.
In spite of the merits described above, the sintered body obtained by the conventional reaction sintering method acquires high density and strength with difficulty as compared with a sintered body obtained by the atmospheric sintering method or the hot press method. It is, therefore, deficient in mechanical properties such as strength and resistance to wear and finds fairly limited utility as a high-temperature structural material. If the reaction sintering method could be improved to fabricate products of suitable high density, the shaped body obtained thereby would also acquire improved mechanical properties, manifest the aforementioned features to the fullest extent, and prove useful as a high-temperature structural material.
The major reason that high density products cannot be obtained by the reaction sintering is that sintering shrinkage scarcely occurs when the structure of the molded material transforms into sintered silicon nitride.
As one means of increasing the density of a reaction sintered body of silicon nitride, there can be cited a method of adjusting the size of Si particles, heightening the molding pressure thereby increasing the density of the resultant-shaped body of Si particles, and subjecting the shaped body to a nitriding reaction.
This method is not completely satisfactory, because the density finally attained has its limit.
Another method developed similarly for the purpose of increasing density involves sintering Si in an atmosphere of inert gas such as Ar gas prior to subjecting the shaped body to a nitriding reaction. This method, however, does not bring about an effective sintering shrinkage. This is because the migration of Si is preponderantly caused by vaporization and surface diffusion and sintering shrinkage is obstructed by voluminal expansion. It has been reported that boron or a compound thereof possesses an ability to repress the vaporization and surface diffusion of Si which is detrimental to the sintering shrinkage of Si and, therefore, serves efficiently in the increase of density. Addition of boron, however, results in degradation of the resistance to oxidation.
A two-stage sintering method has been proposed as a process to improve the above-mentioned defects in reaction sintering. This two-stage sintering method includes preparing a porous silicon nitride sintered product by reaction sintering, impregnating the pores of the sintered product with a compound of Al, Mg, Y, or the like as a sintering promoting agent of silicon nitride, and then sintering the same again to cause sintering shrinkage to finally densify the structure of the product. However, this two-state sintering method has a defect in that the sintered product thus obtained shows the same significant reduction in product strength as in the atmospheric sintering method at a high temperature range above 1000.degree. C. Therefore, this defect eliminats the merit of the reaction sintering itself.
Further, as another method for improving the density and strength of a sintered product, there has been proposed a method of using powdery metallic silicon grains as the starting material and densifying the product through shrinkage during sintering of the grains. However, this method has not yet actually been successful, because a film of silicon oxide (silica) formed on the surface of the powdery silicon grains causes obstruction of the sintering between silicon grains. Although the use of H.sub.2 gas may be considered for removing the silica film, the reaction between H.sub.2 gas and the silica film takes place only at a high temperature above 1200.degree. C., where the sintering between silicon grains proceeds rapidly, to cause such a significant sintering shrinkage as to eliminate the open pores necessary for the subsequent nitriding reaction in the molded product. Thus, since the nitriding reaction itself is inhibited, this method cannot be utilized.
Therefore, since the optimum temperature for controlling sintering between silicon powder is about 1100.degree. C., it is necessary that the silica film on the surface of the powdery metallic silicon grains be eliminated before the temperature is increased to that level.